1. Field of the Invention
The present invention relates to a light guide panel in which light introduced from a side end surface emanates from a front or back surface, the light guide panel being used in a plane illuminator apparatus, and more specifically to plane illuminator apparatus for use in illumination of a display using a transmission or a reflection type liquid crystal device.
2. Description of the Prior Art
A plane illuminator apparatus used as the so-called backlight for a liquid crystal is adapted such that light from a light source is guided into a transparent light guide panel from a side end surface of the same. The light uniformly emanates from a front or back entire surface area of the light guide panel by making use of the reflection of the light in the light guide panel. Considering the characteristics of a liquid crystal display for which a plane illuminator apparatus is used, it is particularly important that the plane illuminator apparatus emit light uniformly over the whole of the apparatus. It is also preferred that the apparatus be thin plate-shaped as a whole and that power consumption of a corresponding light source be reduced to the utmost.
To achieve such requirements, a conventional plane illuminator apparatus enjoys a structure in which there are superimposed two prism sheets one on the other. A light reflection sheet is provided on a back surface side of a light guide panel and a plurality of isosceles triangle pillar-like shaped prisms are parallely arranged on the front surface side of the light guide panel such that these prisms cross perpendicularly to each other in their longitudinal directions. More specifically, light emanating from the back surface side of the light guide panel is forced to again impinge the light guide panel with the aid of the light reflection sheet and light emanating from the surface of the light guide panel is converged with a pair of the prism sheets to ensure highly bright illumination light.
There is further known a light guide panel in which light incident on the light guide panel is intended to be uniformly distributed. For these innumerable dots with the size from several hundred micrometers to several millimeters, white colored ink is printed on the back surface of the light guide panel. For preventing these dots from obstructing the view, a light diffusion sheet is interposed between the light guide panel and the prism sheet. The light diffusion sheet disperses light emanating from the front surface of the light guide panel.
The light emanating from the front surface of the light guide panel suffers ordinarily from uncontrollable directivity which is dependent upon the physical properties of the light guide panel. A direction where maximum brightness is obtained and a desired direction of a view is obtained are practically not coincident with each other. Therefore, simple superposition of the prism sheet on the front surface of the light guide panel causes the light from the light guide panel to emanate only from one slope side of the prism, so that light intensity with uniform distribution is not obtained.
Prior art in which dots formed with white colored ink are printed on the back surface of the light guide panel, and light propagating in the light guide panel is diffused, suffers from occurrence of absorption loss of the light. Further, in order to prevent these dots from becoming conspicuous, it is necessary to jointly use the light diffusion sheet so that most of the light passing through the light diffusion sheet becomes diffusion light to result in sharp reduction of brightness. As a result, a bright light source with a greater intensity of radiation must be used. It is further quite impossible to control the propagation direction of the light.
The Conventional prism sheet is capable of converging the diffusion light from the light guide panel. Light emanating from the light guide panel, however, contains components which are not perpendicular to the surface of the light guide panel and are generally inclined in the direction where they are separated from the light source. Thus, the Conventional prism sheet cannot deflect the light emanating from the front surface of the light guide panel in a desired direction, i.e., perpendicularly to the front surface of the light guide panel.
The Conventional plane illuminator apparatus uses two prism sheets superimposed on the light guide panel in addition to the diffusion sheet. This large number of parts makes it impossible to reduce the thickness thereof as a whole. Severe optical loss happens owing to the diffusion sheet and owing to reflection on an interface between the two prism sheets. As such, is necessary to use a bright light source with a greater intensity of radiation. This causes a factor of preventing the whole of the plane illuminator apparatus from being made compact and the apparatus from being made low power consumption.
A reflection type liquid crystal display has higher image contrast than a transmission type liquid crystal display to ensure an excellent quality image, but it requires illumination by outdoor daylight and hence it can not be used at night or in a room and the like without any illumination. Accordingly, a transmission type liquid display in which a backlight source can be incorporated is solely used as a display mounted on a portable small-sized computer.
There are situations where effective illumination is required for a limited area without use of a large-scaled illuminator instrument without limiting to such a reflection type liquid crystal display.
For such a requirement, a method is used conventionally in which an object to be observed is directly illuminated with an LED or a midget lamp.
Recently, a small-sized illuminator apparatus has been designed for reflection type liquid crystal display comparable with the backlight source for use in a transmission type liquid crystal display such that a reflection type liquid crystal display excellent in view of image quality is usable at night or in a room without any illumination.
However, the conventional illuminator apparatus has the problem that it is difficult for the apparatus to achieve uniform illumination because the apparatus illuminates an object to be observed from the side thereof or obliquely from the front with light from an LED or a midget lamp. It is therefore difficult to apply the apparatus to a reflection type liquid crystal display.
Further, there is a possibility that when an illumination light source is located between an observation point and an object to be observed, the illumination light source interferes with a liquid crystal panel and hence the whole of the object fails to be observed.